In the past decades, the continuous trend in the development of electronic devices is to minimize the sizes of the devices. While the current generation of commercial microelectronics are based on sub-micron design rules, significant research and development efforts are directed towards exploring devices on the nanoscale, with the dimensions of the devices often measured in nanometers or tens of nanometers. Besides the significant reduction of individual device size and much higher packing density compared to microscale devices, nanoscale devices may also provide new functionalities due to physical phenomena on the nanoscale that are not observed on the microscale.
For instance, electronic switching in nanoscale devices using titanium oxide as the switching material has recently been reported. The resistive switching behavior of such a device has been linked to the memristor circuit element theory originally predicted in 1971 by L. O. Chua. The discovery of the memristive behavior in the nanoscale switch has generated significant interests, and there are substantial on-going research efforts to further develop such nanoscale switches and to implement them in various applications.
The original nanoscale resistive switch utilizing titanium oxide as the switching material is a two-terminal device, with the switching material sandwiched between two electrodes that may be segments of two intersecting nanowires. When a relatively high switching voltage is applied to the two electrodes, the strong electrical field causes drifting of oxygen vacancies in the switching material. The redistribution of the oxygen vacancies in the switching material alters the resistance of the switching device. In this way, the device can be switched to ON or OFF states that correspond to high and low resistance values. The state of the switch can be determined later by applying a sensing voltage to the electrodes to sense the resistance of the device. The sensing voltage is much lower than the switching voltage required to cause ion drifting so that the state of the switch is not altered by sensing.
In the two-terminal nanoscale switching device, the two electrodes are used for both switching and sensing operations. For some applications, however, it may be desirable to have separate connections for device switching and sensing. For instance, in some applications, it may not be possible to both set the state of the switch and sense the device using one set of electronics. Moreover, the voltage and current requirements for setting the switch state may be much larger than those for state sensing, and better device control and greater design flexibility may be obtained by having separate connections for dedicated switching and sensing circuits. To that end, it has been proposed to use a three-terminal switching device that has a third electrode for the main purpose of applying the switching voltage to set the operational state of the device. Nevertheless, there has been no suitable design of such a nanoscale three-terminal switching device that can be fabricated in a practical way or allows integration of multiple three-terminal devices in an array such as in a nanowire crossbar design.